INFLUENCE OF THE SEDIMENTATION PROCESS ON THE RAŠA RIVER MOUTH ON THE OUTFLOW REGIME OF COASTAL SPRINGS
RUBINIĆ JOSIP
OŽANIĆ NEVENKA
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ABSTRACT
The paper discusses the influence of the sedimentation process at the Raša river mouth on the outflow regime of several coastal springs important for the water supply of Istria, among which some are subject to salination. The reason of existence of a number of high-yielding springs in this area may be explained by the fact that the Raša river valley, whose canyon part is cut deep into the upper Cretaceous limestones of the Istrian karst plain during the Holocene transgression is the lowest drainage base for surface and ground water in the broader part of Istria. The valley bottom is covered by deep impermeable alluvial Holocene sediments, and the above-mentioned ascending karst springs upper in the zone of contact between the karst background and the valley, or the sea. The basic limestone rock mass in the Raša river estuary is found at depths over 80 m, within the reach of intensive development of karst phenomena. The sedimentation process in the river mouth has resulted in building up of sediment layers at the beginning of the estuary where the springs are situated, and these layers reach to the depth of some seventy or more meters.
The paper also discusses the process of formation of the valley zone and the effects of this process on the water regime of the springs involved in these changes. Depending on the location-distance of individual sources from the sea and their position in relation to the sediment, the influence of the sea on salination of source water is noted. The paper discusses the influence of the position of the sediment on the hydraulic mechanism of water outflow from the sources. As examples, three lowest lopated important sources in the Raša river catchment area have been selected - the spring Rakonek as the most downstream source in the Raša valley in which the influence of the sea is not felt, the spring Fonte Gaja-Kokoti where the influence of the sea is felt only in extreme hydrologic conditions, and the spring Blaz, in which salination is the regular seasonal problem.
As the natural sedimentation processes affect the outflow regime of the sources and protect them from salination, they may, in the upstream part of the Raša river, serve as a model for future measures for protection of downstream springs from salination.
Key words: sedimentation of the river mouth, coastal karst springs, salination, the
Raša river, estuary, Istria
INTRODUCTION
The Istrian peninsula comprises about 3.160 sq.km., out of which the major part belongs to the Istrian Country (2.880 sq.km.) with the population of about 200.000. In summer, it is necessary to add tourists to this figure which come to approx. 21 million tourist days. With regard to the growing trend in water demand, as well as to the observed falling trend of the discharge balance of the existing sources (Rubinić et al. 1998), it is necessary, for further development of the Istrian region, along with preservation of the water quality on the existing intakes, to provide additional water resouces for water supply.
The area of Istria consists of Jurassic, Cretaceous and Paleogenic strata of calcareous and clastic formation, with prevailing typical karst features. The major characteristics are the large water collection zones in the mountain area, and very complex conditions in source zones, in contact with impermeable barriers consisting of clastic layers, or under backwater action of the sea. Although the Istrian peninsula is not a strictly defined hydrological or hydrogeological unit, it is hydrographically separated from the rest of the mainland, it may be regarded as a separate and complex, but practically an integrated aquifer with its own regularities of behaviour. Such theory about mutual connections of aquifers in the Istria area is supported by the results of underground water tracings performed so far.
The major drainage basins in Istria are the catchment areas of the Mirna, Dragonja, Raša and Boljunčica rivers. The largest springs in Istria are situated along the borders of their valleys. Sedimentation processes are present on the river mouths of all watercourses. The valley parts of the river mouths of the Mirna and Dragonja, formerly deeply indented into the mainland, were formed by sedimentation. On the other hand, with regard to the size of the Raša Bay, out of the total of 12 km, sedimentation affects only about 3 km of the upstream part of the bay. Sedimentation process is evident also at the mouth of the Boljunčica river, which was redirected towards the Plomin Bay only in 1932.
The sedimentation process is directly related to the changes in the outflow mechanism of the coastal karst springs, and the changes of hydraulic conditions for fresh water outflow in the zone of contact with the sea. While in the Mirna valley stable marginal conditions have been formed, and the springs Gradole and Bulaž do not show salinity, in the lower part of the Raša catchment area, where the largest still non-utilized water resources in Istria are situated, the outflow conditions of some sources is stabilized by sedimentation, while some of the springs in the estuary are exposed to sporadic or permanent salination.
SEDIMENTATION PROCESS AND HYDROTECHNICAL WORKS IN THE RAŠA RIVER MOUTH
The Raša valley was formed by the Raša river, a torrential watercourse with the catchment area of about 450 sq.km. The mean annual discharge at the mouth is 4,8 cu.m./sec., out of which the major part is contributed to the water balance by the overflow from the sources in the valley (about 2/3). The estuary of the Raša river was formed by submerging of the river valley. After ending of the process of rising of the sea level some 6000 years ago, the process of filling of the estuary, mainly by fine particles of suspended material, was initiated. The work (Juračić et al., 1995) gives, based on the results of two bathymetric surveys of the Raša Bay bottom, the estimate that the quantity of load sedimented in the period from 1938 to 1968 was 1.450.000 cu.m., or about 80.000 tons per annum. The dinamics of sedimentation in the Raša estuary is mentioned in the by Benac et al.(1991) to have caused advancing of the river mouth by approx. 4 km in the past 240 years. The same work also determines that, in the investigated part of the Raša Bay (upstream from Trget) the bedrock - limestones mass - is found at depths below 80 m. In the area around Trget, the greatest depth to the bedrock has been determined, of 86 m below the sea level, while the greatest sea depth in this area is only 15 m. From this, it may be seen that the greatest thickness of the sediment layer is about 70 m. The sediment layer becomes thinner towards the margins of the bay, resulting in smaller depths to the bedrock. Going downstream, the water depth in the Raša Bay gradually increases, from 30 - 35 m in the central part of the bay to 44 m at the entrance into the bay.
It should be noted that two opposite processes are present in the Raša Bay. On one hand, there is global trend of rising of the sea level, which in the last two thousand years is 1,9 m, and on the other hand there is the still more dominant trend of filling of the bay by sediment brought by Raša river. The sedimentation decreases going away from the Raša river mouth, as shown in Fig.1. illustrating, along with the general situation, the average increment of sediment in various sections of the Raša Bay in the period from 1938 to 1968. Downstream from the area around the spring Blaz, in the above 30-year observation period no shallowing has been noticed (Juračić et al., 1995), or the increment was too small in the comparatively short historical period to be quantified.
In the zone of the Raša river mouth, extensive hydrotechnical works were carried out in the period from 1935 to 1939, including the regulation of the Raša course and construction of the hydroamelioration system canal network of the area about 4 sq.km. The direct influence of the sea reaches several kilometers upstream along the Raša river channel and the canals of the amelioration system. This sometimes results in increased salinity of fresh water in water supply sources, e.g. the spring Fonte Gaja - Kokoti.
Although the process of sedimentation in the Raša Bay contributes to the increase of the area of fertile land and, changing the outflow conditions from sources protects them from salination, it is, at the same time, a very serious impediment for sea traffic of the Bršica harbour. The situation is similar approx. 15 km away in the Plomin Bay due to large quantities of sediment load brought by the Boljunčica river after construction of the tunnel. It has been determined that after construction of the tunnel, in the period from 1997 to 1998 the quantity of sediment in the Plomin Bay was about 300.000 cu.m. of sediment were dredged out. This sediment was deposited into the sea directly in front of the Plomin Bay. Simulating the natural sedimentation processes in the Raša Bay valley it would be more acceptable to use it for separation of the coastal sources from direct contact with the sea, in order to prevent their salination.
CHARACTERISTIC OF WATER REGIME OF SOURCE IN THE ZONE OF THE RAŠA RIVER MOUTH
The mechanisms of karst springs in the zone of possible sea influence vary a great deal, and in addition to general hydraulic principles of interaction of fresh and salt water on the premises of the Ghyben-Hertzberg Law they are greatly dependent on the local geological and hydrogeological conditions. According to Šegota (1968), 25.000 years ago the sea level on our coast was 96,5 m lower than at present, and the depth of karstification is considerably far below the present sea level. This has resulted in the expansion of the zone of possible contact between fresh and salt water in the karst systems open towards the sea. Also, the possibility of salination of sources has increased. According to Bonacci (1997) there are three possible ways of protecting the karst springs exposed to salination:
1- Construction of grouting screens (curtains) isolating fresh karst aquifers from influenece of the sea;
2- Artificial raising of the water level in spring, by the construction of a dam ahead of the karst spring;
3- Abstraction of fresh water in the karst massif away from the contamination zone.
With respect to the varying conditions of karst springs outflow and geological and hydrological characteristics of the aquifers, none of the above ways may be in advance considered advantageous in comparison with the others. It is necessary, for each individual source subject to salination problems, to study the regional hydrological circumstances and the local conditions. Fig.2. shows a scheme of interaction between salt and fresh water in the lower Raša area regarding the analyzed springs. Their basic hydrological features and assessment of conditions regarding the contact between spring and sea water for the purpose of their protection from salination, are given below.
RAKONEK SPRING
Rakonek spring is a typical karst ascending spring on the contact between the karst limestone massif and the valley sediment of the Raša river (Fig.1c.). However, the settled valley sediment towards the approx. 500 m distant river channel gives a sufficiently wide insulating layer, and the influence of the sea is not felt at the spring, even when water level was reduced by pumping below the sea level. The salinity ranges from 14 to 25 mg/l. annual water abstraction reaches 7 million cu.m. The elevation of the spring overflow is 3,9 m a.m.s.l., and the water levels, depending on the hydrological conditions and abstraction regime, range between -0,6 and +5,0 m a.m.s.l. On the average, the water level each year is below the overflow sill for 120 days. The mean annual discharge is 0,412 cu.m./sec., the minimum is 0,200 cu.m./sec., and the maximum is 2 cu.m./sec. It should be noted that the Rakonek spring has a limited outflow capacity, and high discharges are released on the Grdak spring approx. 600 m away.
FONTE GAJA-KOKOTI SPRING
The source consists of two hydrologically and hydraulically connected springs at mutual distance of 270 m, located on the margin of the limestone rock massif and Raša valley. Between the valley soil with the amelioration system and limestone background, there is the lateral canal which evacuates the overflow from the source, but also allows circulation of the sea in the vicinity of the spring (Fig.2c.). The annual abstraction rate is about 3 million cu.m. The usual annual minimum yield of these two springs is about 0,200 cu.m./sec., and the maximum is about 5-6 cu.m./sec. The elevation of both springs overflows is comparatively low, and during long dry season the water level in the springs is around 0,5,m a.m.s.l., and maximum at 4,0 m a.m.s.l.
In normal conditions the salinity is between 15 and 68 mg/l, but on several occasions considerable increases of salinity were noted, endangering water supply. During a long dry period and increased abstraction, the salinity at the Kokoti intake gradually increased up to 150 mg/l with further growing trend, and the abstraction had to be reduced in order to stabilize the salinity. In September 1998, extreme salinity was recorded for two days on the Fonte Gaja intake (742 mg/l) in conditions of sudden water inflow after a long dry period. This may be explained by the increased hydraulic pressure in the zone of mixing between salt and fresh water, caused by the inflow of large quantities of rainwater, and the residual ”pockets” of salt water trapped in the ground were forced out in places where the hydraulic resistance was the lowest, in the given case directly through the spring. With regard to this, a possible model of protection of this source from salination would be shifting of the lateral canal in order to avoid the contact between sea water and water in the Fonte Gaja-Kokoti source zone.
BLAZ SPRING
Blaz spring is situated 3,5 km downstream from the Raša river mouth, in the zone where the sedimentation processes of the Raša river are still present. The sea in the outflow zone of the spring is comparatively shallow - the depth of 5 m is 200-250 m far from the coast. Outflow of fresh water on the contact between the limestone bedrock and the sea is noticed in the source zone approx. 300 m wide. The main spring Blaz flows out at the altitude of approx. 0,5 m above sea level, and therefore it is also exposed to the influence of the contact between salt and fresh water, being regularly salinated during the dry part of the year (Fig.2a.). The salinity at the main spring is between 25 and 13.000 mg/l. The mean annual discharge is about 1 cu.m./sec., and the minimum, depending on the particular year, between 0,2 and 0,5 cu.m./sec. In several occasions the possibility of water abstraction from this source and its protection was investigated, but also so far without any results.
In order to achieve the higher pressure to prevent sea penetration into the source zone, the elevation of the spring outflow was raised by about 0,5 m, and the attempt was made to abstract water by about 250 m of tunnel in the spring background before its contact with the sea, and the plans also included construction of the injection curtain, which has not been done due to the high costs involved and to uncertain results. All these measures treated the problem of source protection from salination on a local basis, which with respect to the wide outflow zone, could not result in satisfactory protection. Therefore, this paper proposes a completely different approach to protection of the Blaz spring from salination - by simulation of the natural sedimentation process of the Raša Bay in a wide zone of influence. This is supported by the fact that sedimentation process already exists in the sea in front of the source, and that the sediment layer above the limestones bedrock already impedes the communication between the source and the sea, though only in part. Forming of a plateau in the part of the bay in front of the Blaz spring and its outflow zone, the possibility of this contact with the sea would be reduced or even completely eliminated. This would, in case that such contact is not fully prevented, allow other usual measures to prevent salination (raising of the outflow level and inje ction curtain). The material for the plateau would not be a problem, as it could be provided by dredging of the parts of the Raša and Plomin Bay for harbour facilities.
CONCLUSIONS
In the wider Raša river zone, there are several important springs, and their use is sometimes difficult (spring Fonte Gaja-Kokoti) or even impossible (Blaz) due to salination. Natural sedimentation processes in the outflow zones of the sources and zones of their contact with the sea provide natural protection against salination (Rakonek). Such protection model - separation of sources from the sea influence by sediment material may be the basis of the concept of future measures to secure additional water resources in the Raša river mouth zone. With regard to the need for reconstruction of the hydroamelioration system of the Lower Raša, it is necessary, along with hydrotechnical conditions related to agricultural requirements, to consider the possibility of reducing the influence of the sea on the existing sources of water supply.